I always admire someone who has written something and wants to make sure it is as effective as it can be, so I commend you for that.
Your sentence covers a lot of ground, and I assume it is a thesis/purpose statement for an essay.
After observing and studying the rise of the Communist and Fascist regimes of Hitler and Stalin (from the early- to mid-20th century), Orwell uses 1984 as a medium to expose the brutal reality behind the megalomaniac's propaganda for the promise of a utopia.
My first observation is that it has a lot of information; however, not all of it has to be included in this one statement. For example, saying that Hitler and Stalin are Communist and Fascist seems redundant. Of course it is true and it could and should be discussed in the essay; however, it is not necessary in this sentence.
Another example is the parenthetical expression. It is common knowledge that this is the time frame for these two regimes, so again it can be omitted here but of course used elsewhere in the essay or paper.
Now the sentence reads this way:
After observing and studying the rise of the regimes of Hitler and Stalin, Orwell uses 1984 as a medium to expose the brutal reality behind the megalomaniac's propaganda for the promise of a utopia.
More pruning (editing) can be done to make it an even more effective sentence (such as eliminating either observing or studying); however, the issue I have is the word megalomaniac. To whom, exactly, does this word refer? As written, it is a singular noun (one megalomaniac), which means it cannot refer to both Hitler and Stalin, though I suppose it could refer to just one of them (which seems odd). It surely does not refer to Orwell, so I wonder who this word references.
You may have something different in mind, but this is the next iteration:
After studying the rise of the regimes of Hitler and Stalin, Orwell uses 1984 as a medium to expose the brutal reality behind the use of propaganda which promises utopia.
Obviously, this is your thesis, so be sure it says everything you need and be sure my changes did not distort or change your intention. This really is just an edited (streamlined) version of the thesis you wrote, and perhaps it will work for you. I have attached an eNotes resource for writing a persuasive/argumentative essay in case you might find that helpful.